Can you tell?

Situation 1:

Imagine 4 people were put like the picture above. A wearing yellow hat and is sitting behind the wall and cannot see the other 3 people. B were sitting in front of the wall wearing red hat. C is wearing a yellow hat and D is wearing a red hat. All of them did not know what color is the hat that they are wearing. They were wearing their hats while they are blindfolded. They were regulated not to tell their friends the colors of their friends hat or asking them for theirs. They cannot turn their back. They can only look straight from the position of their sittings. What they only know is 4 of them are wearing 2 red hats and 2 yellow hats. Each of them were asked to determine what color are their own hat and gave a theory to strengthen their answers. A did not know what color is his hat. He also did not know each of the other 3 persons hat. B is in the same situation of A because he only can stare the wall. Whereas C can see what color of hat is B wearing. D can see both B & C hats. So, maybe he has a bigger chance on building a theory for his hat color or maybe not.

Situation 2:

A person will come to each one of them and ask them base on their situation, can they determine a theory of what colors of their hats are they wearing. Lets name him The Host. All four of them are able to hear each of the persons conversation between the host. The host will ask A. “Do you know what color of your hat?”. Of course A will answer no because he is sitting in front of the wall and cannot see the other 3 person. The host will ask B the same question. B will answer no also. The host then go to C & D asking the same question. But both of them asnwer no. After 5 minutes, the host ask four of them the same question again. But this time there is only one person have the answer. Can you tell which person is that and why? Common guys, put your answer in the comments below. Lets see who can answer this riddle.